Dipcoating methods have been used for years to produce shells for investment casting. The first step in the conventional dipcoating process involves preparing slurries comprising suspensions of refractory materials, such as ceramic particles. The second major step requires serially dipping patterns, such as wax patterns or patterns comprising polymeric materials produced by stereolithography, into one or more refractory slurries. This builds up refractory material on the outside surface of the pattern to form a shell in the shape of the pattern.
The slurry-forming step requires preparing multicomponent slurries which include both a flour (i.e., refractory materials) and a binder system. With known methods the binder system includes at least one inorganic binder. The slurry system can be either an aqueous-based system, or a nonaqueous-based system. For aqueous-based systems, the inorganic binder generally comprises nanometer-sized colloidal silica.
Binder particles in a slurry may permanently bond together which, over a period of time, renders the slurries unusable. Binder particle bonding is facilitated by particle drying at the air-slurry interface. Several factors contribute to binder particles coming to air-slurry interfaces. For example, production slurries generally are open to the air and are continuously being mixed. Moreover, air often is continuously bubbled through the slurries to agitate the suspensions. These actions produce air-slurry interfaces, and hence facilitate binder-particle bonding. Once patterns or patterns coated with refractory layers are dipcoated they are removed from the slurry. Excess slurry is allowed to drip off of the shell and back into the container housing the slurry. This also promotes the formation of large air-slurry interfaces. The consequence is that slurries lose their useful properties over time, which is referred to in the industry as "slurry aging" or just "aging".
Besides aging due to the formation of slurry-air interfaces, the different refractory materials used in the slurry also may cause differential slurry aging. For example, yttria-based slurries age very rapidly and may become completely useless within less than a few hours. Alumina-based slurries age slower, and in a similar environment may last a few months.
In aqueous slurries, the soluble species originating from each component of a slurry may specifically adsorb on the surface of silica binder particles. By modifying the surface properties of silica particles, this process can cause aging over time. The degree of interaction among different components of a slurry and the binder particles differs from one slurry to another. For example, Horton et al.'s U.S. Pat. No. 4,947,927 illustrates that in yttria-based slurries, slurries age and become useless in less than a few hours. Yasrebi et al.'s U.S. Pat. No. 5,407,001 teaches that if the yttria used to form yttria-based slurries is fused with a few percent of zirconia, then the solubility of yttria decreases and slurries containing such materials may be used for a period of greater than about a week. The extent of interaction between slurry components and binder is even less in slurries made with less soluble materials, such as alumina, zircon, or fused silica. These slurries can be prevented from aging for extended periods of time, such as months or even longer than a year if the slurries are kept in sealed containers.
In non-aqueous slurries silicon alkoxides often are used as binders. An increase in the pH of non-aqueous slurries as the result of dissolution of components normally reduces the lifetime of these slurries even more dramatically than for their aqueous counterparts. Similar to aqueous systems, the aging behavior of these slurries may differ depending on the amount of dissolution of components in the slurry. For example, yttria-based non-aqueous slurries age very rapidly and may become useless in less than a few hours, whereas alumina-based slurries age slower and in a similar environment may last a few weeks. On the other hand, in some slurries, such as zircon slurries, the pH increase is quite small and slurries may last for many months. Other non-silicon based alkoxides normally go through hydrolysis and condensation reactions very rapidly and age prematurely in the presence of even less reactive refractories such as zircon. For this reason, non-silicon based alkoxides have not been used as binders in the investment casting industry.
Each of the factors discussed above contributes to causing slurry aging. Methods continually are sought to reduce or eliminate slurry aging. One reason for this is that the slurries used to make shells commercially preferably are made in large quantities. It may take many months to use the entire contents of each slurry vessel. If the slurries do not have long shelf lives, they must be disposed of periodically.
Besides slurry aging, there are other problems associated with the conventional shell making process. Such problems include, but are not limited to, reactions between the metal and the shell during the casting process (referred to herein as metal-shell reaction) and undesirable mechanical properties in the shell itself. Concerning metal-shell reaction, the casting process requires first melting the metal and thereafter filling investment casting shells with the molten metal. Some of the metals cast by investment-casting methods are highly reactive. Titanium is an example of such a metal. Furthermore, the reactivity of most metals and metal alloys increases at the high temperatures at which metal articles are cast. The refractory particles used to form the shells therefore may react with the molten metal introduced into the shell. For example, silica binders can react with the molten metal. Silica binders therefore present two considerations. First, the binding properties of silica are the primary reason why conventional shell-making processes work as well as they do, and hence silica binders are an important component in most shell-making compositions. On the other hand, silica causes slurry aging over time. Moreover, silica generally is the least refractory component in a shell, and is the material most likely to react with the metal being cast.
There have been attempts to substitute higher-refractory non-silica binders for silica binders in investment casting slurries. These attempts have proven impractical for use in commercial processes. One reason for this is excessively rapid slurry aging.
The mechanical properties of a shell also are affected by the methods used for their production, as well as by the materials used to construct the shells. For example, green strength, which refers to the strength of the shell before it is fired (i.e., heated to elevated temperatures before being used to cast metals) is an important consideration. An adequate green strength is required to construct a shell around a pattern. An adequate green strength also is important to prevent the shell from cracking as the pattern about which the shell is constructed is separated from the shell. The green strength of a shell appears to correlate with the binder concentration in the slurry. For example, an increase in the concentration of colloidal silica increases the green strength, at least to a point. However, there is a maxima in the curve for green strength versus silica-binder concentration. This is referred to herein as the critical binder concentration, i.e., the binder concentration corresponding to the maxima in the green strength versus binder concentration curve. Shell strength generally decreases when the binder concentration exceeds the critical binder concentration.
High-temperature shell stability is another mechanical property of interest. If the shell deforms at high temperatures, such deformities will be manifested in the metal article being cast. Deformation of the shell at high temperatures is referred to herein as "shell creep" or just "creep." It currently is believed that the main cause of shell creep is the silica binder. Silica is an amorphous material which does not crystalize easily, and this is believed to contribute to shell creep at high temperatures.
Another problem associated with the investment casting process is separating the shell from the cast metal article once the casting process is complete. This is referred to as knockout. Obviously, the easier the shell is to remove from the cast metal article, the better. The need for adequate green strength directly opposes the need to have facile separation of the shell from the cast metal article. That is, for most shells increased green strength makes it more difficult to separate the shell from the cast metal article.
Based on the preceding discussion it is apparent that there is a need for methods and compositions that reduce the aging of slurries used in the investment casting industry. There also is a need to accomplish the goal of reduced slurry aging while maintaining or improving the important physical characteristics of the shell.